Bandpass filter including monolithic crystal elements and resistive elements

ABSTRACT

A bandpass filter circuit includes a pair of monolithic crystal filter elements and a resistance-capacitance network, which may be a lattice network, connected between the crystal filter elements. The crystal filter elements are dual-coupled resonators which have a pair of resonator spots formed on a quartz wafer, and the resonators have relatively well defined bandpass characteristics extending above and below their resonant frequencies and provide an abrupt change in attenuation at the limits of the bandpass frequencies. The resistance-capacitance network used with the crystal filter elements acts to relocate the pole frequencies so that the characteristic curve of the complete filter closely approximates a Gaussian shape in the vicinity of the bandpass. The bandpass filter circuit thus formed is particularly adapted for use in a mobile receiver where extraneous undesired signals may occur in the crystal filter elements resulting from impulse type signals, such as that produced by spark discharge of the ignition system, and these undesired signals are greatly reduced or eliminated.

Unite States Patent 1191 Dailing et al.

[ BANDPASS FILTER INCLUDING MONOLITHIC CRYSTAL ELEMENTS AND RESISTIVEELEMENTS [75] Inventors: James L. Bailing, Glen Ellyn; Corwin E.Livenick, Hickory Hills; Stan- Iey Malinowski, Park Ridge, all of Ill.

[73] Assignee: Motorola, Inc., Franklin Park, Ill. [22] Filed: Dec. 16,1970 [21] Appl. No.: 98,722

[52] US. Cl. ..333/72, 3l0/9.8, 330/l74 [51] Int. Cl. ..H03h 7/06, H03h7/08, H0311 9/00 [58] Field Of Search ..333/ 72, 70; 330/174; 334/40; 3l0/9.8

[56] References Cited 1 UNITED STATES PATENTS 1,969,571 8 1934 Mason..333/72 3,430,163 2/1969 Schindall 3,593,218 7/1971 Wood 3,633,1341/1972 Barrows et al....

2,959,752 11 1960 KOSOWsky.....

2,459,019 1 1949 Ol-Ieedene...

3,222,622 12/1965 Curran et al ..3 10/8 1 x FOREIGN PATENTS ORAPPLICATIONS FRONT END 14 1 Apr. 10, 1973 OTHER PUBLICATIONS Waren,Gerber, Curran: Application of Energy Trapping to Quartz-Filter Design(Frequency Control Symposium), Frequency, May-June 1965, pp. 26-33.

- Primary Examiner-Rudolph V. Rolinec Assistant Examiner-Marvin NussbaumAttorney-Mueller & Aichele [57] ABSTRACT A bandpass filter circuitincludes a pair of monolithic crystal filter elements and aresistance-capacitance network, which may be a lattice network,connected between the crystal filter elements. The crystal filterelements are dual-coupled resonators which have a pair of resonatorspots formed on a quartz wafer, and the resonators have relatively welldefined bandpass characteristics extending above and below their reso-'nant frequencies and provide an abrupt change in attenuation at thelimits of the bandpass frequencies. The resistance-capacitance networkused with the crystal filter elements acts to relocate the polefrequencies so that the characteristic curve of the complete filterclosely approximates a Gaussian shape in the vicinity of the bandpass.The bandpass filter circuit thus formed is particularly adapted for usein a mobile receiver where extraneous undesired signals may occur in thecrystal filter elements resulting from impulse type signals, such asthat produced by spark discharge of the ignition system, and theseundesired signals are greatly reduced or eliminated.

12 Claims, 8 Drawing Figures PATENTED APR 1 0 I975 SHEET 1 [1F 2 ATTYS.

PA TENTEDAPR I 05975 FIG 5 RECEIVER FRONT END SHEET 2 OF 2 RECEIVERFRONT END RECEIVER FRONT END RECEIVER FRONT END Inventors JAMES L.DAILING CORWIN E. LIVENICK STANLEY MALINOWSKI ATTYS.

BANDPASS FILTER INCLUDING MONOLITHIC CRYSTAL ELEMENTS AND RESISTIVEELEMENTS BACKGROUND OF THE INVENTION Reference is made to US. Pat. No.3,633,134, issued Jan. 4, 1972 to Richard G. Barrows and William G.Ahillen, which discloses and claims a crystal bandpass filter circuitrelated to the bandpass filter circuit of the present invention.

This invention relates generally to crystal filter circuits, and moreparticularly, to bandpass filter circuits having dual-coupled resonatorswhich are used as intercoupling stages between IF amplifiers, or thelike.

In providing bandpass filter circuits using monolithic crystal filters,it is very difficult to eliminate the objectionable affects of ringingcaused by undesired pulse signals. Although crystal filter elementsprovide a well defined passband, which may be of the order of 4 to 30kHz for a crystal element having a resonant frequency of about 5 to 30MHz, the steep or sharp sides of the response curve near the nose of thecurve, and the abrupt change therein, have been found to result inundesired ringing effects. This is a particularly important problem inconnection with radio communication equipment for use in automobileswhere ignition spark discharge is a major cause of extraneous signals,which can cause ringing in the filter and be heard in the speaker of theradio receiver. Such signals can also result in false operation when thereceiver is used with coded signals. 7

Summary of the Invention An object of this invention is to provide animproved crystal filter circuit wherein the affects of extraneousimpulses applied to the crystal filter elements-are substantiallyminimized.

Another object of this invention is to provide an improved crystalfilter circuit which allows for shaping of the passband of any chosenfilter characteristic (Butterworth, Chebyshev, Legendre, ImageParameter, etc.) to a shape approaching the desired Gaussian curve inthe passband while maintaining the original frequency attenuationcharacteristics outside the passband.

A further object of the invention is to provide an improved bandpassfilter circuit including monolithic crystal elements for sharpselectivity and one or more resistive-capacitive networks to change thefilter characteristic in the vicinity of the passband so that thedesired Gaussian characteristic is provided.

A feature of this invention is the provision of a bandpass filterincluding a lattice resistance-capacitance network or its derivativecoupled between monolithic dual-coupled crystal filter elements with thenetwork acting as part of the filter circuit and, as such, modifying thecharacteristic curve of the crystal filter elements to the desiredessentially Gaussian curve in the passband.

Briefly, the bandpass filter circuit of this invention is adapted toreceive an intermediate frequency (IF) signal, such as that produced atthe output of a radio receiver front end. The IF signal is applied to afirst dual-coupled resonator which may be formed on a single quartzcrystal wafer and forms part of the bandpass crystal filter circuit. Thesignal is coupled through a resistance-capacitance network to a secondand similar dual-coupled resonator which provides the filter output. Theresonating portions of each dual-coupled resonators can be at the sameor a slightly different frequency, depending upon the design selected,but in either case the frequency of the mesh in which each resonatingportion is connected is the same, thereby providing the desiredrelatively narrow and well defined passband. Although each resonatingdevice is here illustrated as being a dual-coupled resonator on a singlequartz wafer, which may be flat or contoured, it will be understood thateach device may have three or more resonating areas formed on a singlewafer, if desired, and that more than two resonating devices can be usedin a filter circuit.

In accordance with this invention, the output of the first dual-coupledresonator is coupled to the input of the second dual-coupled resonatorthrough a resistance-capacitance network which is selected to modify thecharacteristic curve of the crystal filter circuit. By so changing thecharacteristic curve in the vicinity of the passbandto a Gaussian curve,undesired transient pulse-type responses caused by extraneous signalsare greatly reduced or eliminated.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a schematic diagramillustrating the equivalent circuit arrangement of a crystal filtercircuit constructed in accordance with this invention;

FIG. 2 illustrates the bandpass curve of the filter of FIG. 1;

FIGS. 3 and 4 illustrate the impulse response characteristic of filtersbefore and after, respectively, the bandpass characteristic curve ischanged to a Gaussian: shape in the passband; and

FIGS. 5, 6, 7 and 8 illustrate various forms of crystal filter circuitswhich can be constructed in accordance with this invention.

DESCRIPTION OF THE INVENTION AS ILLUSTRATED IN THE DRAWINGSThroughoutthe several views of the drawings, like reference numeralsindicate similar or like components. Also, the components of the severalembodiments are shown queued from an output circuit of a receiver frontend to the input of an amplifier, but it will be understood that anycomponent arrangement may be used.

Seen in FIG. 1 is a schematic diagram illustrating the equivalentcircuit construction of a bandpass filter circuit including dual-coupledresonators and a resistance lattice network interconnected with theresonators in accordance with this invention. Here the front end 10 ofan FM receiver, which may include an RF amplifier, local oscillator andmixer, produces an IF frequency at its output which will be translatedthrough the crystal filter circuit of the invention. The PM receiver maybe of any desired frequency range suitable for mobile use. The signalwhich is developed at the output of the receiver front end 10 isimpressed across a resistor 12 and an inductance element 21'. This IFsignal from the receiver front end 10 includes desired signals within arange of frequencies and undesired signals above and below this range offrequencies. It is the desired signals within the range of frequencieswhich are to be selected by the crystal filter circuit of thisinvention. To this end, a pair of resonators 14 and 16, with aresistive-capacitive network 15 therebetween, are interposed in thesignal path from the receiver front end to an amplifier circuit 18. Theresonators 14 and 16 may include quartz crystal wafers or wafers ofother piezoelectric material such as ceramic. The resonators are hereshown by their equivalent circuit components and they preferably takethe form of dual-coupled monolithic crystal filter elements.

The equivalent components of the crystal filter element 14 include aninput capacitance C,, which may include the electrode capacitance, andwhich is shunted by the external inductance 21 which may have a valuethat will effectively tune out the input capacitance C Series capacitorsC and C and series inductors L and L provide a signal path through thefilter element, and inductor L, is the effective shunt inductanceelement. An output capacitance C, also includes electrode capacitance.Similarly, the equivalent circuit of the crystal filter element 16 isshown with a plurality of inductors and capacitors in substantially thesame manner and designated by the same reference numbers as element 14.The elements are reversed for a symmetrical configuration. The output ofthe crystal filter element 16 is shunted by an external inductor 35,which tunes out the capacitance C Thesignal translated through thecrystal filter circuit so formed is applied to load 42, which may be theinput impedance of possible resistance-capacitance coupling arrangementscan be formed by changing the values or eliminating certain ones of therespective resistors and/or capacitors. That is, a series resistancecoupling network, a pi network, or an L network, or any combinationthereof,

can be formed merely by changing the relative values of the componentsforming the lattice network 15. Also, networks of other configurationscan be derived from the lattice network.

:In accordance with one aspect of this invention, the resistancevalues,,regardless of the type of network ultimately formed by thelattice network 15, are selected to change the shape-of thecharacteristic curve of the bandpass filter circuit from what it wouldbe to closely approximate a Gaussian shape in the passband. For example,a pair of monolithic crystal filters of the'type illustrated, whenconnected directly in series, would have a natural characteristic curveas illustrated by the broken line portion 17a of the curve 17 of FIG. 2.The

relatively sharp or'abrupt corners at the lower end of the curveillustrate a substantially uniform minimum attenuation for thefrequencies in the band extending below and above the center frequency,and then maximum attenuation for all frequencies beyond these values.However, this particular characteristic tends to cause substantialringing to occur at the output of the filters as a result of impulsetype signals. This is illustrated in FIG. 3 by the pulse including aseries of lobes designed generally by reference numeral 19, whichdiminish in amplitude with increasing time, and the ringing within eachlobe is at or near the center frequency of the crystal filter. Becauseof the repetition of such pulses and the consequent transient response,unwanted signals can be heard in the audio output of the receiver andmay constitute a major noise disturbance in the receiver output.

However, it has been found that by proper selection of the resistancenetwork between the crystal filter elements l4 and 16, thecharacteristic curve (FIG. 2) of the filter circuit can be changed, 7 asindicated by reference numeral 17b, to a Gaussian curve which issymmetrical on either side of the vertical axis. By so changing theshape of the characteristic curve of the filter circuit, the subsequentlobes of each pulse will have substantially reduced amplitude, as shownin FIG. 4. Thus, the audible noise at the receiver output is reduced.This is shown diagrammatically by a single lobe 21 of ringing which isimmediately followed by a sharply damped lobe and thereaftersubstantially no response at all. This single pulse 21 has a relativelyshort time duration and is substantially inaudible, at least as comparedto the previous pulses 19 as shown in FIG. 3.

When the crystal filter elements 14 and 16 are dualcoupled monolithiccrystal devices, the resonating areas on the quartz crystal may beformed to be resonant at slightly different frequencies to compensatefor variations in input capacitance or inductance values of thecircuitry connected thereto. Although there can be a slight differencein resonant frequency of the resonating areas due to the filterdesign'chosen, the resonant frequencies of the meshes of the circuit inwhich they are connected is the same.

FIG. 5 illustrates one specific form of this invention and again showsthe frontend 10 of an FM receiver 7 which provides an IF signalgenerally within a range of frequencies defined by the passband of thefilter circuit involved. As previously stated, the IF signal may includesignals above and below the desired passband frequency. To eliminate allfrequencies which fall outside the particular passband involved, thebandpass filter, including the pair of monolithic crystal filterelements 14 and 16 and the single resistor 32, is provided for selectingthe signal prior to amplification by amplifier circuit 18. I

In the embodiment illustrated in FIG. 5, the IF signal is applied byresistor 12 to a capacitor 20 and an inductance element 22 which form atuned circuit. The inductance element 22 has a tap 22a thereof coupledthrough a second inductance element 24 to the first crystal filterelement 14, here shown diagrammatically as a quartz body with electrodesformed thereon. Shunt capacitors 26 and 28 may be discrete components,but may be the internal interelectrode capacitance of the crystal filterelement 14, such as shown by capacitance C of FIG. 1, and in this casethe value of such capacitance is determined by the characteristics ofthe particular crystal filter element involved. The crystal filterelement 14 is preferably formed of a single flat, or contoured, crystalbody with electrodes diffused or deposited thereon to form a pair ofresonating portions within the crystal body. Two pairs of electrodes orterminals are provided on the crystal body so that the resonatingportion of each resonator, i.e. crystal filter element, is that portionbetween the diametrically opposed terminals. For example, the crystalfilter element 14 has an input terminal 14a for receiving signals and van output terminal 14b for passing signals of the desired resonatingportion to the other by the interaction of the equivalent inductance andcapacitance, as shown in FIG. 1.

A resistor 32 is coupled between the output of the filter element 14 andth e input of the filter element 16,

with the ends of the resistor 32 connected to shunt capacitors 28 and34. The capacitors 28 and 34 provide the proper signal coupling betweenresonators 14 and 16. The signal is then applied to the input terminal16a.

of the filter element 16 and received at the output terminal 16b in thesame manner as described above with regard to filter element 14.'It willbe noted that the single resistor 32 is a derivative circuit formed fromthe lattice network shown in FIG. 1.

. The signal translated through this passband circuit is then developedacross an inductance element 36 and a pair of series connectedcapacitors 38 and 40, and across a resistor 42 which is connected onlyacross the capacitor 40. The signal is then applied to the amplifier 18and therefrom to a detector or discriminator circuit,

not shown. Here also the capacitor 34 maybe the inherent capacitance ofthe input terminals of the crystal element 16.

The crystal filter circuit of this invention minimizes I the response'ofthe crystal filter elements 14 and 16 to extraneous impulse typesignals, while maintaining proper frequency attenuation. Basically, thecrystal filter circuit disclosed provides means for shaping a passbandcharacteristic curve of any chosen filter filter elements 14 and 16while simultaneously maintaining the original frequency attenuationcharacteristics in the reject band, also maintaining symmetry offrequency'attenuation about the center frequency,

and maintaining the original design mesh frequencies.

The crystal filter circuit design is chosen to approximate the frequencybandpass characteristics desired, and this may or may not have passbandripple, and may or. may not have staggered crystal frequencies. Thebandpass width is initially designed wider than the desired bandpasswidth, since the shaping of the nose of the characteristic curvenecessary to minimize the impulse response will cause a correspondingdecrease in bandpass width, so that the ultimate bandwidth is achieved.The indiscriminate introduction of the resistance network between theappropriate crystal filter elements 14 and 16 will result in a shiftingof some of the pole frequencies and/or a non-symmetry in the frequencyattenuation characteristics from the originally designed crystal filterelements. This action will cause a strong discriminator output when thefilterdiscriminator combination is subjected to impulse type inputsignals, which is an undesired result. Therefore, the resistancecoupling network must be designed to minimize these effects, i.e. tohave a symmetrical Gaussian curve characteristic.

In addition, resistor 32, or another form of the re sistance network 15of FIG. 1, will decrease the quality factor Q (de-Qing) of theassociated crystal elements 14 and 16, thus resulting in a moretolerable filter arrangement since it is less susceptibleto loadimpedance variations. That is, approximately plus or minusSO percentvariation from the normal design impedance value can be toleratedwithout any marked passband ripple changes, or any marked non-symmetryin frequency attenuation characteristics which would otherwise manifestitself if the quality factor Q of the elements 14 and 16 were maintainedat their natural high level, as for example, in the order of 20,000.

The resistive-capacitive network is an integral part of the crystalfilter design and may be external to the crystal elements orincorporated on a crystal chip design as an integral'unit. Furthermore,the network may be provided in the form eitherof a thin film or thickfilm deposited on a quartz substrate. The techniques of crystal filtercircuitconstruction can be applied to coupled crystal resonators havingmorethan two coupled resonators on a single quartz wafer.

FIG. 6 illustrates an alternate circuit arrangement wherein severalmodifications have been made, Here the crystal filter element 14 isshunted with a capacitor- 44 and the crystal filter element 16 isshunted with a capacitor 46. The use of these capacitors acts tosteepen' the sides of theresponse curve. A tank circuit 48 is connectedat the output of crystal filter element 16 and is shunted by a fixedresistor 50. In this embodiment the resistance network between the twocrystal filter elements 14 and 16 is provided by a resistance L- padcircuit comprising a first resistor 52 and a second resistor 54 whichhave their values selected to produce the desired change in the passbandcharacteristic curve.

Referring now to FIG. 7, there is illustrated still another alternateform of this invention. Here the output from the receiver front end 10is applied across an inductance-capacitance network includingan'inductor 56 and a capacitor 58. The signal is then coupled through acapacitor 60 to a second inductancecapacitance network including aninductor 61 and a capacitor 62. Crystal filtering action is thenachieved by a crystal filter element 64 which is provided with ashunting capacitor 65 between the separate resonating portions formedwithinthe body thereof. The output terminal side of the crystal filterelement 64 is shunted by a resistor 67 to provide for part of theGaussian curve characteristic to be formed. In this embodiment, a secondcrystal filter element 68 has the input terminal side thereof connectedin series with a fixed resistor 69 which also serves to provide foranother part of the Gaussian curve characteristic. A second fixedresistor 71 is connected in series with the output side of the filterelement 68 and in this instance, the bottom terminals of the filterelement are not tied together. The IF signal is coupled through thecrystal filter element 68 and then applied across a resistor 72 andthence to a thirdcrystal filter element 73, which is shunted by'ductance-capacitance network including an inductor 76 and'a capacitor77 'at the output of the crystal filter circuit. The IF signal sodeveloped is then applied to the integrated circuit amplifier 18 througha coupling capacitor 78.1nthis embodiment, the resistance net workwhichforms the Gaussian curve-includes resistors 67,69, 71 and-72associatedwith the crystal filter elements 64, 68 and 73. Also, each crystalfilter element is a. dual-coupled resonator similar to those describedin FIGS. and 6. i v

Referring nowto FIG. 8, there is seen still another alternate'embodiment of this invention. Here the receiver front end appliesthe IFsignal to an inductance-capacitance network including an inductor 80 anda pair of cap'acitors'8l and 82 connected in parallel therewith, thissignal coupling being made through a resistor 79. A first crystal filterelement 83 passes the desired IF frequency signal and is shunted by acapacitor 84. The output terminal 'of the element 83 has a resistor 86connected in series therewith. A second. crystal filter element 87receives the IF frequency signals, and this crystal filter element has'afixed resistor 88connected in series with the input portionthereof. Inthis embodiment a resistor 89 is'conne'cted in parallel relation withthe'two resonating portions and their associated series connectedresistors 86 and 88 respectively-To make this parallel connection,

resistor 89 is connected to a common line 91 between the crystal filterelements 83 and 87, and together with ,the series connected resistors 86and 88 serve to form the desired Gaussian curve. The signal is alsocoupled through a shunt capacitor 90 to an inductor 92 and a capacitor93. The IF signal is ultimately deliveredto the wide band integratedcircuit amplifier 18 through a coupling capacitor 94. aThejcenterfrequency of the bandpass filter-circuit disclosed herein maybe at any desiredfrequency'and filters have been successfullyconstructed for frequen- 0f M P l 111111956856 0f 1 second resonator forcoupling signals from said second MI-lz filter, the characteristic curveportion l7 bjof FIG. 2 will be about 6 DB down at about 5.5 to 6 kl-laabove and below, the center frequency, and about 110 DB down at 26 kHzabove and below the center frequency.

By 'way of example, in the crystal filter circuit designed for'5.26 MHz,the pair of resonating portions may be of different frequencies. In thecrystal filter circuit designedto pass 11.7MI-Iz, the pair of resonatingportions may be resonant at the'same frequency. However, in either casethe characteristic curve is changed to approximate a Gaussian shape inthe passband by the proper value of the resistance coupling network. Thefollowing values are given by way of example and are typical for theequivalent circuit shown in FIG.

1 with regard to the two frequencies which have been discussed:

Values for Values for Reference No. 5.26MHz H.7MI-Iz Inductor L, 0.66H0.25H v Inductor L. l30uH 50uH Inductor L, .066H .025H Inductor 21 200uH60uI-l Inductor 35 200uH 60uH Capacitor C, 4.0pf 3.0pf Capacitor C, .0 l4pf .007pf Capacitor C, 0.0l4pf .007 t Capacitor C. 4.0pf 3.0p CapacitorC, 1.0pf 1.0pf Capacitor C 1.0pt' 1.0pf

Capacitor C 1.0pf 1.0pf Capacitor C 1.0pf f 1.0pf Resistor R, 1.5K ohms330 ohms ResistorR, 33.0K ohms 18.0K ohms Resistor R, 1 33.0K ohmsl8.0K' ohms Resistor R 1.5K ohms 330 ohms Resistor 12 1.0K ohms 1.0Kohms Resistor 42. 3.3K ohms 5.6K ohms It may be desired to eliminate theresistance R, and

thecapacitor C so that all of theshunt elements can be connected to asingle reference or ground potential In such case, the valueof resistorR should bedoubled in value.

What has been describedis a simple and effective crystal filter circuitarrangement with aresistivecapacitive network coupled between monolithiccrystal filter elements acting as an integral part of the filter circuitand the component values are selected to provide the proper Gaussiancurve and passband characteristics to reduce. the time response withregard to extraneous signals and thereby reduce the effect of undesiredringing orother response at the output of the crystal filter.

I. A bandpass filter circuit for passing frequencies within apredetermined'passband, including in combina'tion, first and secondresonators each including a single crystal wafer and electrode meanscooperating passband-of thefilter circuit, each of .said first and. I

second resonators acting to couple signals through said wafer thereoffrom said first resonating portion to said second resonating portionthereof, a coupling circuit connected from said pair of electrodes ofsaid; second resonating portion of said first resonator to. said pair ofelectrodes of said first resonating portion of said resonating portionof said first resonator to said fir st "resonating portion of saidsecond resonator, said Gaussian shape to reduce undesired impulse-typesignal transients, and means coupled to said pair of electrodes of saidsecond resonating portionsof said second resonator for receiving thesignal translated through the bandpass filter circuit. A y

2. The bandpass filter circuit of claim 1 wherein said crystal wafers ofsaid first and second resonators are made of quartz, and said resistancemeans is a single resistor connected in series between one electrode ofsaid second resonating portion of said first resonator and one electrodeof said first resonating portion of said I second resonator.

one electrode of said first resonating portion of said second resonator,and with said second resistor connected in shunt relation with saidelectrodes of said second resonating portion of said first resonator.

4. The bandpass filter circuit of claim 1 wherein said resistance meansincludes a resistor connected in series with said electrodes of saidsecond resonating portion of said first resonator.

5. The bandpass filter circuit of claim 1 wherein said resistance meansincludes a resistor connected in series with said electrodes of saidfirst resonating portion of said second resonator.

6. The bandpass filter circuit of claim 1 wherein said resistance meansincludes a first resistor connected in series with said electrodes ofsaid second resonating portion of said first resonator, and a secondresistor connected in series with said electrodes of said firstresonating portion of said second resonator.

7. The bandpass filter circuit of claim 1 wherein said resistance meansincludes a first resistor connected in series with said electrodes ofsaid second resonating portion of said first resonator, and a secondresistor connected in parallel with said first resistor and itsassociated series connected resonating portion.

8. The bandpass filter circuit of claim 1 wherein said resistance meansincludes a first resistor connected in tion of said second resonator,and a second resistor connected in parallel with said first resistor andits associated series connected resonating portion.

- 9. The bandpass filter circuit of claim 1 wherein said resistancemeans includes a first resistor connected in series with said electrodesof said second resonating portion of said first resonator, a secondresistor connected in series with said electrodes of said firstresonating portion of said second resonator, and a third resistorconnected in shunt relation with said first and second resistors andtheir respective associated series connected resonating portions. v

10. In a bandpass filter circuit for passing frequencies withina'predetennined passband which includes first and second resonators eachhaving first and second resonating portions with a pair of electrodes,input signal means coupled to the electrodes of the first resonatingportion of the first resonator for applying therebetween signalsincluding signals which fall within said predetermined passband of thefilter circuit, a coupling circuit for coupling signals from theelectrodes of the second resonating portion of the first resonator tothe electrodes of the first resonating portion of the second resonator,and means coupled to the electrodes of the second resonating portion ofthe second resonator for receiving the signal translated therethrough;the improvement wherein each of the first and second resonatorscomprises a single quartz crystal wafer and first and second pairs ofelectrodes thereon forming first and second resonating portions, witheach pair including electrodes on opposite sides of said wafer, each ofsaid first and second resonators acting to couple signals through saidwafer thereof from said first resonating portion to said secondresonating portion thereof, and wherein said coupling circuit includesresistance means and capacitance means, with said resistance means beingconnected in series circuit with said fpair of electrodes of said secondresonating portion 0 said first resonator and with said pair ofelectrodes of said first resonating portion of said second resonator,said resistance means having a value such that the characteristic curveof the filter circuit in the passband is of substantially symmetricalGaussian shape to reduce undesired impulse-type signal transients.

11. The combination of claim 10 wherein said capacitance means includesa first capacitor connected across said electrodes of said secondresonating portion of said first resonator, and a second capacitorconnected across said first resonating portion of said second resonator.

12. The combination of claim 11 wherein said resistance means is asingle resistor connected in series between one electrode of said secondresonating portion of said first resonator and one electrode of saidfirst resonating portion of said second resonator.

1. A bandpass filter circuit for passing frequencies within apredetermined passband, including in combination, first and secondresonators each including a single crystal wafer and electrode meanscooperating therewith to form at least two separate resonating portionseach including a pair of electrodes on opposite sides of said wafer,input signal means coupled to said pair of electrodes of the firstresonating portion of said first resonator for applying therebetweensignals including signals which fall within said predetermined passbandof the filter circuit, each of said first and second resonAtors actingto couple signals through said wafer thereof from said first resonatingportion to said second resonating portion thereof, a coupling circuitconnected from said pair of electrodes of said second resonating portionof said first resonator to said pair of electrodes of said firstresonating portion of said second resonator for coupling signals fromsaid second resonating portion of said first resonator to said firstresonating portion of said second resonator, said coupling circuitincluding resistance means connected in series circuit with said pair ofelectrodes of said second resonating portion of said first resonator andsaid pair of electrodes of said first resonating portion of said secondresonator, said resistance means having a value such that thecharacteristic curve of the filter circuit in the passband issubstantially of symmetrical Gaussian shape to reduce undesiredimpulse-type signal transients, and means coupled to said pair ofelectrodes of said second resonating portion of said second resonatorfor receiving the signal translated through the bandpass filter circuit.2. The bandpass filter circuit of claim 1 wherein said crystal wafers ofsaid first and second resonators are made of quartz, and said resistancemeans is a single resistor connected in series between one electrode ofsaid second resonating portion of said first resonator and one electrodeof said first resonating portion of said second resonator.
 3. Thebandpass filter circuit of claim 1 wherein said resistance means isformed of first and second resistors connected in an L-padconfiguration, with said first resistor connected in series between oneelectrode of said second resonating portion of said first resonator andone electrode of said first resonating portion of said second resonator,and with said second resistor connected in shunt relation with saidelectrodes of said second resonating portion of said first resonator. 4.The bandpass filter circuit of claim 1 wherein said resistance meansincludes a resistor connected in series with said electrodes of saidsecond resonating portion of said first resonator.
 5. The bandpassfilter circuit of claim 1 wherein said resistance means includes aresistor connected in series with said electrodes of said firstresonating portion of said second resonator.
 6. The bandpass filtercircuit of claim 1 wherein said resistance means includes a firstresistor connected in series with said electrodes of said secondresonating portion of said first resonator, and a second resistorconnected in series with said electrodes of said first resonatingportion of said second resonator.
 7. The bandpass filter circuit ofclaim 1 wherein said resistance means includes a first resistorconnected in series with said electrodes of said second resonatingportion of said first resonator, and a second resistor connected inparallel with said first resistor and its associated series connectedresonating portion.
 8. The bandpass filter circuit of claim 1 whereinsaid resistance means includes a first resistor connected in series withsaid electrodes of said first resonating portion of said secondresonator, and a second resistor connected in parallel with said firstresistor and its associated series connected resonating portion.
 9. Thebandpass filter circuit of claim 1 wherein said resistance meansincludes a first resistor connected in series with said electrodes ofsaid second resonating portion of said first resonator, a secondresistor connected in series with said electrodes of said firstresonating portion of said second resonator, and a third resistorconnected in shunt relation with said first and second resistors andtheir respective associated series connected resonating portions.
 10. Ina bandpass filter circuit for passing frequencies within a predeterminedpassband which includes first and second resonators each having firstand second resonating portions with a pair of electrodes, input signalmeans coupled tO the electrodes of the first resonating portion of thefirst resonator for applying therebetween signals including signalswhich fall within said predetermined passband of the filter circuit, acoupling circuit for coupling signals from the electrodes of the secondresonating portion of the first resonator to the electrodes of the firstresonating portion of the second resonator, and means coupled to theelectrodes of the second resonating portion of the second resonator forreceiving the signal translated therethrough; the improvement whereineach of the first and second resonators comprises a single quartzcrystal wafer and first and second pairs of electrodes thereon formingfirst and second resonating portions, with each pair includingelectrodes on opposite sides of said wafer, each of said first andsecond resonators acting to couple signals through said wafer thereoffrom said first resonating portion to said second resonating portionthereof, and wherein said coupling circuit includes resistance means andcapacitance means, with said resistance means being connected in seriescircuit with said pair of electrodes of said second resonating portionof said first resonator and with said pair of electrodes of said firstresonating portion of said second resonator, said resistance meanshaving a value such that the characteristic curve of the filter circuitin the passband is of substantially symmetrical Gaussian shape to reduceundesired impulse-type signal transients.
 11. The combination of claim10 wherein said capacitance means includes a first capacitor connectedacross said electrodes of said second resonating portion of said firstresonator, and a second capacitor connected across said first resonatingportion of said second resonator.
 12. The combination of claim 11wherein said resistance means is a single resistor connected in seriesbetween one electrode of said second resonating portion of said firstresonator and one electrode of said first resonating portion of saidsecond resonator.